Which of the following is an example of internal exposure?

• A. External exposure example: X-ray radiographs.
• B. External exposure example: Background radiation.
• C. Surface contamination from a spill.
• D. Ingestion or inhalation of radionuclides (internal exposure).

Answer: (D)

Internal exposure occurs when radioactive material gets inside the body, delivering radiation from within the tissues and organs. When radionuclides are inhaled or ingested, they deposit in places like the lungs, liver, or bones and continue to emit radiation from inside, which can cause dose to those specific organs over time. Ingestion or inhalation are the classic routes that create this internal dose.

External exposure, by contrast, comes from radiation sources outside the body, such as X-ray radiographs or ambient background radiation, where the radiation passes through the body from outside. Surface contamination can lead to external exposure or be a pathway to internal exposure if material is ingested or inhaled, but by itself it describes contamination on surfaces rather than entry into the body.